Technology is available for correcting excessively slow heart rates (bradycardia) using implantable devices, commonly referred to as pacemakers, which deliver microjoule electrical pulses to a slowly beating heart in order to speed the heart rate up to an acceptable level. Also, it is well known to deliver high energy shocks (e.g., 180 to 360 joules) via external paddles applied to the chest wall in order to correct excessively fast heart rates, and prevent the possible fatal outcome of ventricular fibrillation or certain ventricular tachycardias. Bradycardia, ventricular fibrillation, and ventricular tachycardia are all electrical malfunctions (arrhythmias) of the heart. Each may lead to death within minutes unless corrected by the appropriate electrical stimulation.
One of the most deadly forms of heart arrhythmias is ventricular fibrillation, which occurs when the normal, regular electrical impulses are replaced by irregular and rapid impulses, causing the heart muscle to stop normal contractions and to begin to quiver. Normal blood flow ceases, and organ damage or death may result in minutes if normal heart contractions are not restored.
Because time delays in applying the corrective electrical treatment may result in death, implantable pacemakers and defibrillators have significantly improved the ability to treat these otherwise life threatening conditions. Being implanted within the patient, the device continuously monitors the patient's heart for treatable arrhythmias and when such is detected, the device applies corrective electrical pulses directly to the heart.
Pacemakers and defibrillators that apply corrective electrical pulses externally to the patient's chest wall may be used to correct such life-threatening arrhythmias but suffer from a drawback insofar as it may not be possible to apply the device in time during an acute arrhythmic emergency to save the patient's life. Such treatment is needed within a few minutes to be effective, and the chance of survival begins to diminish rapidly after one minute.
Consequently, when a patient is deemed at high risk of death from such arrhythmias, electrical devices often are implanted so as to be readily available when treatment is needed. Alternatively, such patients are kept in a hospital where corrective electrical therapy is generally close at hand. Long term hospitalization, however, is frequently impractical due to its high cost, or due to the need for patients to engage in normal daily activities.
There also are many patients susceptible to heart arrhythmias who are at temporary risk of sudden death. For example, patients who have suffered a myocardial infarction and have low ejection fraction are at substantial risk of tachyarrhythmias for several weeks thereafter. Such patients generally are hospitalized but could be discharged earlier if there were a practical means to protect them from life threatening arrhythmias. Additionally, patients awaiting implantation of an automatic defibrillator may require an external defibrillator to be close at hand, in case they experience a life-threatening tachyarrhythmia. Furthermore, some patients who may benefit from an implantable defibrillator may face an inordinate risk from the surgery required for implanting such a device.
Wearable external defibrillators are known, such as those disclosed in U.S. Pat. No. 5,741,306 assigned to the assignee hereof and hereby incorporated herein by reference in its entirety for all purposes. A wearable defibrillator may provide a patient-worn energy delivery apparatus for imparting electrical therapy to the body of a patient responsive to detection of a treatable condition. An important consideration in the proper operation of the device is accurate sensing of the treatable condition by the apparatus and delivery of the electrical energy to the person's body by electrodes. The electrodes must be placed on the person's body in the correct position in order to effectively perform these functions. It is typically desirable that the electrodes be positioned on both the front and back of the patient in order to provide the most effective electrical therapeutic pulse to the body. Additionally, since the wearable defibrillator is designed to be worn by the patient over extended periods of time, the use of skin-irritating substances commonly used on a more temporary basis to attach electrodes to a patient is typically avoided.